Nielsen television ratings are gathered in one of two ways:
1- Viewer "diaries", in which a target audience self-records its viewing or listening habits. By targeting various demographics, the assembled statistical models provide a rendering of the audiences of any given show, network, and programming hour.
2- A more technologically sophisticated system uses Set Meters, which are small devices connected to televisions in selected homes. These devices gather the viewing habits of the home and transmit the information nightly to Nielsen through a "Home Unit" connected to a phone line. The technology-based home unit system is meant to allow market researchers to study television viewing habits on a minute to minute basis, seeing the exact moment viewers change channels or turn off their TV. In addition to set meters, individual viewer reporting devices, such as people meters, have allowed the company to separate household viewing information into various demographic groups, but so far Nielsen has refused to change its distribution of data of ethnic groups into subgroups, which could give more targeted information to networks and advertisers.
